A typical submersible pump assembly includes a downhole electrical motor and a pump separated by a seal section. Often the pump is a centrifugal type, although progressive cavity types are also employed. The pump assembly is usually suspended on a string of tubing that extends into the cased well. The pump discharges well fluid up the tubing.
When the pump shuts down, either planned or unplanned, fluid may flow back down through the tubing and out the intake of the pump, possibly spinning the pump in reverse. This occurs until the level of fluid in the tubing equals that in the tubing annulus surrounding the tubing. The downward flow through the pump may bring debris in the tubing back into the pump, causing damage on later restarting. Also, at times, the tubing string or the pump can pack off or plug after a shut down. This may prevent flow back down the tubing. If the tubing does not drain, the operator may then have to bail out the production fluid before pulling the pump and tubing. Bailing involves running a bailer on a wire line repeatedly down into the tubing, taking time.